Ask for an MFA code at startup when AWS_MFA_SERIAL is set without a role

[ashishjullia]

• `script.sh`: in the branch with IAM keys and no `AWS_ROLE_TO_ASSUME`, prompt for an MFA code when `AWS_MFA_SERIAL` is set and source `mfa.sh` with it. Same prompt and failure behaviour as the role branch.
• The role branch, `dev` with no project, and projects without `AWS_MFA_SERIAL` are unchanged.
• Readme: documents the new startup prompt.

Not tested end to end (needs a real MFA device); `bash -n script.sh` passes.

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